Wireless communication systems are well known in the art. Communications standards are developed in order to provide global connectivity for wireless systems and to achieve performance goals in terms of, for example, throughput, latency and coverage. One current standard in widespread use, called Universal Mobile Telecommunications Systems (UMTS), was developed as part of Third Generation (3G) Radio Systems, and is maintained by the Third Generation Partnership Project (3GPP).
An example UMTS system architecture in accordance with current 3GPP specifications is depicted in FIG. 1. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an Iu interface. The UTRAN is configured to provide wireless telecommunication services to users through wireless transmit receive units (WTRUs), referred to as user equipments (UEs) in the 3GPP standard, via a Uu radio interface. A commonly employed air interface defined in the UMTS standard is wideband code division multiple access (W-CDMA). The UTRAN has one or more radio network controllers (RNCs) and base stations, referred to as Node Bs by 3GPP, which collectively provide for the geographic coverage for wireless communications with WTRUs. One or more Node Bs is connected to each RNC via an Iub interface; RNCs within a UTRAN communicate via an Iur interface.
WTRUs in a UMTS Terrestrial Radio Access Network (UTRAN) can be in either of two modes: Idle or Connected. Based on WTRU mobility and activity while in connected mode, the UTRAN can direct the WTRU to transition between a number of sub-states, e.g., CELL_PCH, URA_PCH, CELL_FACH, and CELL_DCH. User Plane communication between the WTRU and the UTRAN is only permitted while in CELL_FACH and CELL_DCH state. The Cell_DCH state is characterized by dedicated channels (DCHs) in both the uplink (UL) and the downlink (DL). On the WTRU side, this corresponds to continuous transmission and reception and can be demanding on user power requirements. The CELL_FACH state does not use DCHs and thus allows better power consumption, at the expense of a lower uplink and downlink throughput.
The CELL_FACH is well-suited for signaling traffic (for example, the transmission of CELL/URA UPDATE messages), and for applications requiring very low uplink throughput. In CELL_FACH, uplink communication is achieved through a random access transport channel (RACH) mapped to a packet random access channel (PRACH) physical channel. The RACH is a contention based protocol with a power ramp-up procedure to acquire the channel and to adjust transmit power.
Downlink communication is through a shared Forward Access Transport Channel (FACH) mapped to a secondary common control physical channel (S-CCPCH) or through the high speed downlink channel.
Mobility is handled autonomously by the WTRU in CELL_FACH. The currently soft handover does not (as of Release 6 of the standard) exist within CELL_FACH. As such, the WTRU independently takes measurements, and determines when to make cell reselections.
System information during CELL_FACH is read from a broadcast channel (BCH). This information includes the setup details for the uplink RACH, the downlink FACH and the high speed downlink shared channel (HS-DSCH)) channels to be used in CELL_FACH.
Recent work by the standardization bodies has identified reuse of an Enhanced-DCH (E-DCH) in the CELL_FACH state. Enhanced-DCH is a feature that was introduced to increase uplink throughput. The E-DCH operates on a request/grant principle. WTRUs send an indication of the requested capacity they require through a combination of mechanisms, while the network responds with grants to these requests. These grants are generally generated by a Node B scheduler.
At the same time, Hybrid Automatic Repeat Requests (HARQs) are used in connection with the physical layer transmissions. To facilitate the above mechanisms, two new UL physical channels have been introduced, an Enhanced-Dedicated Physical Control Channel (E-DPCCH) for control, and an Enhanced-Dedicated Physical Data Channel (E-DPDCH) for data. Three new downlink (DL) physical channels, two for transmission of grants and one for fast physical layer acknowledgements (Layer 1 ACK/NACK), were also introduced. The Node B, therefore, is permitted to issue both absolute grants and relative grants. Grants are signaled in terms of a power ratio. Each WTRU maintains a serving grant, which it can convert to a payload size. For Release 6 WTRUs, mobility is handled by the network through soft handover and active sets.
In addition to the new channels at the physical layer, E-DCH is also required at the Medium Access Control (MAC) layer, with the introduction of new MAC-e/es protocol entities to handle the Enhanced Dedicated Transport Channel (E-DCH).
One of the concerns with the use of E-DCH in CELL_FACH is the interaction of the uplink procedure with the mobility procedure, in particular, the cell reselection procedure. This procedure can either remain WTRU autonomous or could be network assisted in some way. In both cases, the network and WTRU actions upon a cell reselection need to be defined. On the WTRU side, actions have to be specified to deal with the medium access control entities (MAC-e/es), hybrid automatic repeat request (HARQ) buffers, MAC Transmission Sequence Numbers (TSN), and the like. With respect to the network, a serving radio network controller (SRNC) may need to be made aware when a new enhanced radio network temporary identifier (E-RNTI) has been assigned by a controlling radio network controller (CRNC). The network may also have to deal with releasing the resources in the source cell.
Accordingly, there exists a need for a method and apparatus to address reselection for WTRUs capable of using the E-DCH while in Cell_FACH state.